372 



GEOGEAPHICAL DISTEIBUTION AND 



[Ch. XL., 



F 



Djticidas, are amphibious, and in the evening quit their lakes 



mmnte 



manner 



of fishes to distant waters. In this 



account for the fry of fish appearing occasionally in small 



pools caused by heavy rains. 



GEOGRAPHICAL DISTRIBUTION AI^D MIGRATIONS OF 



TESTACEA. 



The Testacea are a class of animals of peculiar importance 

 to the geologist ; because their remains are found in strata 

 of all ages, and generally in a higher state of preservation 

 than those of other organic beings. 



Some forms are exclusively confined to warm, others to cold , 

 latitudes. Marine currents flowing permanently in certain 

 directions, and the influx at certain points of great bodies of 

 fresh water, limit the extension of many species. Those 

 which love deep water are arrested by. shoals ; others, fitted 

 for shallow seas, cannot migrate across unfathomable abysses. 

 The nature also of the ground has an important influence on 

 the testaceous fauna, both on the land and beneath the waters. 

 Certain species prefer a sandy, others a gravelly, and some 



muddy sea-bottom. On the land, limestone is of all rocks 



01 



most favonrable t( 

 the grenera Helix 



others. Pro- 



fessor *E. Forbes has shown, as the result of his labours 



X- 



n 



M 



regions of depth, each characterised by its peculiar testaceous 

 fauna. The first of these, called the littoral zone, extends 

 to a depth of two fathoms only ; but this narrow belt is 



mor 



than 100 species 



The second region, 



of which ten fathoms is the inferior limit, is almost equally 

 populous ; and a copious list of species is given as charac- 

 teristic of each region down to the seventh, which lies be- 

 tween the depths of 80 and 105 fathoms, all the inhabited 

 space below this being included in the eighth province, where 

 no less than 65 species of shell-fish or moUusca have been 

 taken. The majority of the shells in this lowest zone are 

 white or transparent. Only two species are common o 



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32 

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